Japanese published patent application JP 2001 191 005 A discloses a device for the intermittent application of adhesive to a continuously running body. In said document, a method is disclosed which supplies a hot melt to a nozzle and applies it to a continuously running substrate. It is disclosed that the supply of hot melt to the nozzle is interrupted when no coating is to be produced. By repeating this process, a substrate is intermittently coated.
Alternatively, intermittent coating can be achieved by increasing the coating gap between the nozzle and the substrate with each interruption to the coating. However, the wake of the coating substance, which still occurs, wets the nozzle lips, and this results in an excessively high start edge when the substrate (arrester film) is approached again. A further drawback of this approach is that the movement of the nozzle and the placement of the wetted nozzle lips on the substrate cause vibrations in the substrate, which in turn influence the spacing between the nozzle lips and the substrate. Accordingly, it is not possible to achieve a continuous wet film thickness.
Secondary lithium ion batteries are based on an arrangement of a plurality of electrochemical cells. The electrodes in an electrochemical cell each comprise an active material from which lithium ions are intercalated into the electrodes during operation of the electrochemical cell. A lithium-containing oxide compound, preferably LiCoO2, LiFePO4, LiNi1/3Co1/3Mn1/3O2 (NCM) or LiNi0.8Co0.15Al0.05O2 (NCA), mesoporous titanium oxide, is used as the active material in the negative electrode (cathode), and conductive carbon black, carbon black, graphite or the like is used in the positive electrode (anode).
DE 10 2004 012 476 A1 discloses suitable coating substances and the production thereof, which are hereby incorporated into the present application by reference.
In a coating method proposed herein, the polymer binder required for the cathode and anode coating compound (coating substance) is dissolved for example in 5-10% fluoroelastomer homopolymerizate or copolymerizate in N-methyl pyrrolidone (NMP), and the resulting polymer solution is mixed with the cathode-specific or anode-specific additives, such as metal oxide intercalatable with lithium or carbons intercalatable with lithium (conductive carbon black, carbon black, graphite or the like), and is dispersed. Subsequently, this dispersion is applied to the substrate, in this case the current collectors or a collector body such as foils, strips, meshes or the like, by film-coating.
Alternatively, polyvinylidene fluoride, on the cathode side, and styrene butadiene rubber, on the anode side, may also be used as a binder to improve the mechanical strength, and provided with conductive carbon black to increase the electrical conductivity. The active materials are accordingly applied to the substrate or the metal arrester material in the form of pastes (coating substance) using solvents and using carboxymethyl cellulose as a thickener.
Because of the low electrical conductivity of the porous particulate layer morphologies, the applied electrode layer (wet film) has inadequate discharge rate properties. In order to increase the electrical collection capacity, the applied electrode layer that is applied to the electrically conductive substrate (collector body, electrode) therefore has to be made as thin as possible. Preferably, wet film thickness of the menisci of between 10 μm (high power) and 850 μm (high capacitances) are aimed for.
M. Schmitt et al., “Slot-die processing of lithium-ion battery electrodes—Coating window characterization”, Chemical Engineering and Processing: Process Intensification, Volume 68, June 2013, pages 32-37, and M. Schmitt et al., “Slot die coating of lithium-ion battery electrodes: investigations on edge effect issues for stripe and pattern coatings”, Journal of Coatings Technology and Research January 2014, Volume 11, Issue 1, pp. 57-63, teach a suitable method for coating foil electrodes using a slotted nozzle, and the optimum operating conditions for carrying out this method as regards the geometrical dimensions of a suitable device, and are hereby incorporated into the present application by reference.
In order to obtain exact (film) start edges and (film) end edges for the layers or coating portions, it is already known from DE 10 246 327 A1 to retract the coating substance after blocking (closing) the supply channel; this effect is known as “snuff-back”. Downstream of the blocking means and upstream of the outlet, the coating substance is stopped and moved backwards by a negative pressure counter to the original flow direction.
US 2002/0017238 A1 discloses an alternative, disclosing a device which applies an intermittent coating to a moving substrate, the coating interruption being brought about not by means of closure by a blocking means in connection with snuff-back upstream of the outlet, but rather by a combination of a change in the internal volume of the slotted nozzle in connection with a snuff-back effect. This is brought about by an adjustment unit in nozzle jaws actively moving the flexible element back and forth. The active increase in volume brings about a suction effect, causing the snuff-back to act on the coating substance in the outlet gap. Accordingly, the flow direction and the shear stress τ present in the coating substance are reversed in direction. Before being applied to the substrate, the coating substance is temporarily stored here DE 10 246 327 in the cavity subjected to negative pressure or in the flexible volume. The stored coating substance is then applied during a subsequent coating cycle.
However, it has been shown that, as a result, undesired pressure fluctuations or pressure spikes occur in the system, and lead to faults precisely at the start of the coating process. Pressure spikes occur as a result of the abrupt opening of the blocking means (valve). The reduction in the flexible internal volume, as the coating substance stored in the cavity is supplied to the displaced volume flow again, causes an undefined input flow to form in the nozzle slot. This is because the shear stress τ of the displaced volume flow is in a different direction from that of the partial volume flow held back by the snuff-back. Thus, a change in direction in the shear stress of the held-back partial volume flow first has to be brought about in order to achieve a sufficiently high stationary shear or shear stress τ toward the nozzle outlet. Accordingly, the input flow is not sufficiently pronounced, and undesired transverse or longitudinal stripes form on the coatings produced.
Further, the flow and processing properties of the coating substance for producing electrode layers for Li ion cells are crucial for the configuration of the device and method. Shear speeds {dot over (γ)} (shear rates) of the coating substance of up to 100,000 s−1 are achieved here. The shear speed is calculated from the ratio of the speed difference between two adjacent liquid layers and the spacing therebetween. In mathematical terms, the shear speed is the gradient of the speed field.
This is of crucial importance, precisely for the coating substances used herein, since said substances behave in a pseudoplastic or shear-thinning manner. Thus, a volume flow only occurs under the action of a shear stress τ above a minimum shear stress τf (flow limit).
A further drawback of the devices known from the prior art is that they merely apply sequential coatings intermittently. Simultaneous application of a plurality of layers on top of one another is not provided.